Exhaust gas from internal combustion engines, power plants, industrial furnaces, heaters, diesel engines, and other devices contains nitrogen oxides, carbon monoxide, and unburned hydrocarbons. All of these emissions are hazardous to the environment and are subject to increasingly strict governmental regulation. Hence there is a significant interest in developing improved catalysts and processes and devices for treating exhaust gases to reduce or eliminate these compounds.
One common route for reducing the level of nitrogen oxides, carbon monoxide, and unburned hydrocarbons in exhaust gas, especially the exhaust gas from internal combustion engines, involves the use of one or more catalysts. For example, it is well known that nitrogen oxides (NO2 and NO, collectively referred to as NOx) can be catalytically converted to nitrogen in the presence of a reducing agent, such as ammonia or hydrocarbons, whereas carbon monoxide and unburned hydrocarbons can be catalytically oxidized to carbon dioxide and a mixture of carbon dioxide and water, respectively. In some cases, a single catalyst system, generally known as a three-way catalyst, can be used to simultaneously reduce NOx, oxidize carbon monoxide, and oxidize unburned hydrocarbons in an exhaust stream, whereas in other cases different catalysts can be used to treat different toxic components of the exhaust.
Known exhaust gas treatment catalysts include nano-clusters of a precious metal, for example, platinum, dispersed on a high surface area support material, such as a metal oxide. More recently, focus has been directed towards catalyst systems in which the level of precious metal is reduced or eliminated, often by the use of supported multi-component metal alloy or oxide compositions, such as mixed oxides of, for example, copper, iron, nickel, cobalt, cerium, and/or zirconium combined with an alumina-based support material. In general, however, these catalyst systems are produced by conventional wet chemistry techniques (impregnation, precipitation, etc), which are typically time-consuming multi-step processes and which are not highly engineered with regard to homogeneity, material interface, structural stability, composition, porosity and other properties. For example, using traditional wet chemistry, it is often difficult to produce finely and homogeneously distributed complex multi-component materials attached to a high surface area support. This is even more difficult with complex metal oxide supports. Moreover, wet chemistry processes are typically low temperature processes requiring additional heat treatment of the structured particulate product. In addition, the temperature stability of the resultant heat treated catalysts is frequently not acceptable for demanding high temperature applications, such as treatment of exhaust gas emissions.
According to the present invention, a facile and flexible route to the production of the high surface area supports and catalytically active phases required for exhaust gas emission catalysts has now been developed, in which one or more catalyst precursors dispersed in a liquid are introduced into a heated reactor and are chemically converted to particulate catalyst component(s) in a flowing gas stream within the reactor. The process is particularly well adapted to the production of hierarchically structured particles in a single step or a small number of steps. Moreover, the resultant structured particles have improved properties as compared with structured particles produced by conventional wet chemistry techniques. For example, particles produced by these vapor phase conversion processes typically exhibit significantly increased homogeneity, dispersion and surface area for the active phase dispersed on the support. Moreover, this high surface area is generally preserved after exposure to high temperatures, primarily because the particles are exposed to a high temperature environment during their production.